(A) Field of the Invention
This invention relates to the formation of inorganic crystals and more particularly relates to a process for forming such crystals from a solution of a crystallizable inorganic material and a liquid solvent for the material.
(B) History of the Prior Art
In the prior art, inorganic crystals were formed or grown by several methods. One of the methods is by sublimation or vaporization of the compound or a precursor of the material to be crystallized followed by condensation of the vapor of the material at a temperature below the solidification temperature of the material. Such methods usually do not result in the formation of large crystals of the material but rather, due to the rapid crystal formation, generally result in crystals of very small particle size, e.g., on the order of a few microns. Furthermore, such a method and needed equipment for forming pure crystals of any substantial size, e.g., having a breadth of over about 500 microns, such as might be suitable for electronic applications, if possible at all, would be complex and costly. Such a vapor deposition method for forming silicon crystals smaller than desirable, e.g., a maximum dimension of about 100 microns, upon graphite or tin from gaseous silane vapor (silicon precursor) optionally with HCl, is described in an article by Graef et al in the Journal of Applied Physics, Vol. 48, No 9, September, 1977. A somewhat similar process using a silicon halide precursor is described in European Pat. Nos. 0-001-942 and 0-001-943.
Another method for forming crystals of inorganic materials is by fusion or melting of the inorganic material followed by very slowly cooling to form crystals. Such methods can result in crystals of large size, but due to the exceedingly long cooling periods to obtain large crystals, such procedures are avoided when more rapid methods are available.
A third method for the formation of crystals is by dissolving the inorganic material in a solvent for the material followed by either cooling the solution to decrease material solubility until crystals precipitate as a result of solution saturation at the lower temperatures or by vaporization of the solvent until crystals form due to saturation of the remaining solvent by the material, or both. Formation of crystals from solution often have well known disadvantages. In particular, crystal formation, especially for large crystals, is often not as fast as desirable and crystal size, while sometimes fairly large, is for certain materials often not as large as desirable, and such solutions are often subject to supersaturation which retards crystal growth thus necessitating inclusion of crystal seeds to act as nuclei in the formation of the crystals.
The above disadvantages of prior art processes for growing crystals are particularly apparent in the electronics area where large crystals of high purity are desired.